In driving a rotary electric machine by an inverter, a control mode is switched over among a sinusoidal wave current control mode, an overmodulation current control mode and a rectangular wave voltage phase control mode. The rectangular wave voltage phase control mode, which uses one-pulse switching, is required to attain both high output power and size reduction of the rotary electric machine. The rotary electric machine is controlled optimally by switching over the control mode between the sinusoidal wave current control mode, which provides a superior characteristic in a low speed region, and the overmodulation current control mode, which is used in an intermediate rotation speed region.
In the sinusoidal wave current control mode and the overmodulation current control mode, current feedback control is performed to output a pulse-width modulation (PWM) pattern to the rotary electric machine by comparing a command voltage and a carrier wave voltage. In the rectangular wave voltage phase control mode, torque feedback control is performed to output a one-pulse switching waveform to the rotary electric machine in accordance with an electric angle by fixing an amplitude of the voltage to a maximum value and controlling phase.
The switching-over among the three modes, that is, from the sinusoidal wave current control mode to the overmodulation current control mode and from the overmodulation current control mode to the rectangular wave voltage phase control mode are performed based on a rate of modulation or an amplitude of a command voltage corresponding to the modulation rate. Since the command voltage amplitude is fixed in the rectangular wave voltage phase control mode at the time of the switching-over from the rectangular wave voltage phase control mode to the overmodulation current control mode, the time point of switching-over is determined based on a phase of an actual current relative to a command current.
Patent document 1 discloses that a conventional control apparatus for a motor drive system has variations in output torque at the time of switching-over from a PWM modulation control method and a rectangular wave voltage control method. In this apparatus, torque is estimated by calculating power from phase currents Iu, Iv, Iw and a voltage V in the rectangular wave voltage control method, and a torque difference is fed back to a command torque. In the PWM modulation control method, the phase currents Iu, Iv, Iw are converted to a d-axis current Id and a q-axis current Iq, and an Id current difference and an Iq current difference are fed back to an Id command current and an Iq command current. Therefore, both the PWM modulation control method and the rectangular wave voltage control method use the currents Id and Iq determined by a current sensor and a rotation angle sensor thereby to ensure stability of the output torque controllability.
Patent document 2 discloses an AC motor drive control system, in which a control mode is switched over from a PWM current control mode to an overmodulation control mode when an amplitude of a voltage exceeds 1.00 time of a peak value of a reference triangular wave voltage. The control mode is switched over to a rectangular wave voltage phase control mode when the amplitude of the voltage exceeds 1. 27 times of the peak value of the reference triangular wave voltage. The control mode is switched over from the rectangular wave voltage phase control mode back to the overmodulation current control mode when an absolute value of phase of an actual current decreases to be less than an absolute value of phase of a command current.
Low-pass filtering is performed on a measured current, because a d-axis current and a q-axis current include periodic noises and high frequency components. Due to this filtering, the switching-over from the rectangular wave voltage phase control mode to the overmodulation current control mode is sometimes delayed. This delay causes hunting of current phase and unstable control. Therefore, the delay in switching-over from the rectangular wave voltage phase control mode to the overmodulation current control mode is reduced by comparing a required voltage amplitude VR taking into consideration of induced voltages and the like with the peak value of the reference triangular wave voltage and providing an offset value for preventing chattering.
Patent document 3 discloses a motor drive system, in which an excessive voltage is generated inside the motor drive system due to excessive regenerative electric power, when an AC motor generates excessive electric power, which exceeds a level acceptable at a DC power source side, that is, an input side. Therefore, electric power consumption of the AC motor is increased to suppress the regenerative electric power supplied to the input side. In PWM control, an optimum efficiency characteristic line, which connects maximum efficiency points in relation to current phase and output torque, is determined with respect to a current amplitude. A loss increase characteristic line is determined as a set of current operation points, which are current phase-shifted from the optimum efficiency characteristic line, so that the control is performed on the loss increase characteristic line. In a rectangular wave voltage control, a supply voltage to a motor is controlled in respect of only phase and a current phase is fixed. Therefore, a required voltage for the motor is decreased by setting the current operation point to an advanced side in a relation of a supply voltage to a motor (motor interphase voltage) and a current phase in respect of torque, so that motor current is controlled in accordance with the PWM control. Thus, motor drive efficiency is decreased while ensuring torque controllability.    Patent document 1: JP 2007-159368A (WO 2007/066182A1)    Patent document 2: JP 2008-11682A (WO 2008/001524A1)    Patent document 3: JP 2007-151336A (EP 1950878A1)
As described above, the control mode is switched over from the rectangular wave voltage phase control mode to the overmodulation current control mode by determining the switching time point in accordance with the current phase of the actual current relative to the command current. If the switching time point is determined in the advance side in a d-q plane relative to the command current as a reference, the control chatters and current fluctuates at the time of switching over the control mode. For this reason, the switching time point is determined at a retard side in the d-q plane relative to the command current as the reference.
Even if a mode switching determination line is set in the retard angle side relative to the command current, the actual current in the switching determination line deviates largely from the command current, to which the current should be controlled, immediately after the control mode is switched over form the rectangular wave voltage phase control mode to the overmodulation current control mode. Because of this large deviation, a command voltage calculated in the overmodulation current control mode is likely to change suddenly relative to a maximum amplitude, which is the last command voltage in the rectangular wave voltage phase control mode, and a phase at that time. Therefore the PWM pattern actually outputted to the rotary electric machine becomes asymmetric. As a result, disturbance of currents and decrease of torque are caused.